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Ecology and control of downy mildew on cucurbits

Ecology and control of downy mildew on cucurbits

Plant Protection Bulletin 34(2): 149-161

Cucumber downy mildew caused by Pseudoperonospora cubensis was quite widespread and serious on cucumber, melon, muskmelon and loaf in Taiwan. The lesion formed at 4-5 days after inoculation on detached leaves of cucumber and melon under 15, 20 and 25 degree C. Sporulation at 5-6 days under 15 and 20 degree C, 6-8 days under 25 degree C. No lesion formation or sporulation under 10, 30 and 35 degree C. The optimal temperatures for sporangial germination were observed at 10 degree C and 15 degree C after 6 hours of incubation, the percentage of germination were 95.18% and 97.52%, respectively. Germination decreased at higher temperature. Most sporangia produced seven zoospores with an average of 6.7, a few sporangia produced 3 or 11 zoospores. The sporangia of Pseudoperonospora cubensis were incubated at 30 degree C for one day, and then transferred to 15 degree C, additional four hours were necessary to germinate, the germination percentage was 20.87%. As long as at 15 degree C for 24 hrs the germination percentage increased to 63.11%. If the sporangia were incubated at 35 degree C for one day first and then transferred to 15 degree C, another 24 hrs were needed for germination, and the germination percentage was only 7.01%. Even at 15 degree C for 48 hrs, the germination percentage was also increased to 22.42% only. If the infected leaves with sporangia were incubated at 30 degree C for one day then moved to 15 degree C, the sporangia also took 4 hrs for germination. While the infected leaves with sporangia were incubated at 35 degree C, it required 24 hrs for germination. On the other hand, the germinability increased with increasing relative humidity, at the RH of 55.81, 95 and 100% the germination percentage were 29.85, 66.30, 89.02 and 97.53%, respectively. The percentage of germination of sporangium was influenced by agar concentration of water agar medium. When the agar contents were at 2.0% and 3.0% the germination percentage were 99.30 and 96.9%, respectively, after 24 hrs of incubation at 15 degree C. As agarcontent reached 8.0%, no germination was observed. Sporangia did not germinate on the leaf juice of Cucumis melo. When the sporangia were incubated on V-8 juice, it showed that the higher the concentration of V-8 juice, the lower the germinability was observed, and 35% of V-8 juice inhibited, germination. In soil with 30% water content the sporangia did not germinate even after 24 hrs of incubation. After 4 days of incubation, the germination percentage was only 2.66%. Therefore, the germinability increased with increasing water content. The sporangia could survive at least 14 days under 30% of water content in the soil. The downy mildew fungus of cucurbits maintained its infectivity in spore suspension after 6 of 24 hrs preparation, however, after 18 hrs, no infectivity was observed. The relationship of disease severity increment was meteorological factors was analyzed. Which indicated that downy mildew disease was positively correlated with rainfall at 7 and 8-14 days before disease occurrence; but negatively correlated with average relative humidity. Fungicide trials showed that the disease rating was lower on the plot which the seedlings were treated with fungicides iirst and then inoculation was made. Among the fungicides tested, 58% Metalafyl + Mancozeb wp and 75% Chlorothalonil wp showed the best effectiveness.

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